Add support for optional callgrind / cachegrind integration

[TheLostLambda]

I'm not sure if this one falls in scope, but it would be lovely to get the kinds of results you get from https://github.com/iai-callgrind/iai-callgrind (a maintained fork of the original iai), but with divans nice interface / register-anywhere system!

Using these main! macros and listing all of the test function names there really feels primitive after being spoiled by divan!

[gibbz00]

Pretty good context as to why time vs cpu instruction count measurements matter: https://youtu.be/qfknfCsICUM?t=1229 😊